Method of forming a thermal emissivity coating on a metallic substrate

ABSTRACT

A method for coating metallic substrates with high thermal emissivity coatings to prevent failure of the thermal emissivity coatings due to handling and thermal shock by applying an intermediate or base coat of nickel aluminide.

United States Patent [72] Inventor James E. Monroe, Jr.

Timonium, Md.

[21] Appl. No. 697 ,281

[22] Filed Jan. 5, 1968 [45] Patented Nov. 16, 1971 [73] Assignee The United States of America as represented by the United States Atomic Energy Commission [54] METHOD OF FORMING A THERMAL EMISSIVITY COATING ON A METALLIC SUBSTRATE 2 Claims, 2 Drawing Figs.

[52] US. Cl. 117/69, 117/46 FS,117/105.2

51 1111.01 C23c7/00 50 FieldofSearch 117/212, 217, 222, 230, 69, 46 F8, 105.2

[56] References Cited UNITED STATES PATENTS 2,966,430 12/1960 Schrewelius 117/212 3,326,648 6/1967 Provisor 117/230 X Primary Examiner-Reuben Epstein At1orneyRo1and A. Anderson ABSTRACT: A method for coating metallic substrates with high thermal emissivity coatings to prevent failure of the thermal emissivity coatings due to handling and thermal shock by applying an intermediate or base coat of nickel aluminide.

METHOD OF FORMING A THERMAL EMISSIVITY COATING ON A METALLIC SUBSTRATE BACKGROUND OF THE INVENTION Prior practices for systems for nuclear auxiliary power, referred to by the acronym SNAP, have employed thermal emissivity coatings, comprising a thin, oxidation resistant, thermal emissivity layer mechanically bonded to a substrate. To this end, a binder has been employed, which has been cured at a low temperature (less than about 750 F.) to bond the coating to the substrate, or the coating has been applied directly to form a mechanical bond on the substrate as described, for example, in U.S. Pat. No. 3,327,778. While these arrangements have been useful and have accomplished the desired mechanical bond, they have involved an interface connection of a coating layer on a substrate with abruptly different physical characteristics, including difierent coefficients of thermal expansion, that have limited the ability of the mechanical bond to absorb thermal or handling shock. It has thus been desirable to'provide means that avoid these heretofore known limitations. It has additionally been advantageous to provide oxidation resistant high thermal emissivity coatings that can be patched and that can be constituted in such a manner as to tailor the thermal emissivity characteristics of the coating in a broad thermal emissivity spectrum.

SUMMARY OF THE INVENTION This invention was made in the course of, or under a contract with the United States Atomic Energy Commission.

This invention involves a method for providing an intermediate base coat of nickel aluminide and thermal emissivity coatings thereon of E=0.8 or higher. The method and construction involved in this invention utilize standard and well known techniques and apparatus and are highly flexible for a wide range of applications, thermal emissivities, materials and coefficients of expansion. To this end, this invention involves BRIEF DESCRIPTION OF THE DRAWING In the drawings where like elements are referenced alike:

FIG. 1 is a partial schematic cross section of one embodiment of this invention having a plurality of layers of varying thermal emissivity material on a metallic substrate;

FIG. 2 is a partial magnified view of the embodiment of FIG.

DESCRIPTION OF THE PREFERRED EMBODIMENT It is known that flame sprayed or plasma are sprayed metal or metal oxide powders can be applied as coats of varying thickness to a variety of metallic substrate materials. The flame spraying of these materials, comprises feeding the powder granules of standard predetermined particle size through an oxygen hydrocarbon flame of about 3,000 C., as provided, for example, by an oxyacetylene flame spray apparatus operating in accordance with standard metallurgical coating techniques followed in metallurgical and ceramic laboratories. US. Pat. Nos. 2,969,309; 3,270,098; 3,294,698 and 3,318,695 describe the use of flame spraying technique and apparatus in coating a substrate with various materials that are fed in powder form through an oxyacetylene flame. Plasma arc spraying is described on page 512 et seq. of Metals Handbook, 8th Ed., by Am. Soc. for Metals, 1964. This invention hereinafter described utilizes these high temperature spraying systems in which a particular spraying sequence is used in a manner described hereinafter in connection with particular compositions or powders.

In accordance with this invention suitable emissivity materials having emissivities E of at least 0.8 are coated on nickel containing substrates with an intermediate nickel-aluminide emissivity material layer or layers. Suitable emissivity materials are: A1 0 CaOTiO,, CI'gOg-YgOa, Iridium, FeOTiO NiO, Ca'IiO or combinations thereof. The following table illustrates emissivities of various of these and other emissivity materials on various substrates:

TABLE I.EXPERIMENTAL SCREENING EFFORT ()F EMIT'IANCE COATINGS ()N (ANDIDA'IE SIFllS'lltA'lES Emittnncu, ing. (livr-Dnllkli Emittance coating Substrate Environmnntul trl'ntinnnt (2,00l) F.) Application None" 0.84, 0.88 Flume sprayed. Stnhly oxidized in air at 2,100 F. [or 1 hr 0.80 llo. .Nono. 0.76 l)o. 0.54 Do. 0. D0. 0.88 l)o. NiO do (1.81 Do. Iridium black Aluminum. 0.90 Do. ZnO TD nickel 0.56 l)o. Zr0;.. do. 0.45 ho. CaTiO -20% R 0.85 D0. CaTiO -20% B11 d0 0. 80, 0.81 D0. CaTiO Pt-20% R11 Heated for 24 hrs. at 2,400 l". with gruphitiu 0. 02 Do. Hf-27% Ta Pt-20% Rh. Nonv 0.58 Do. CaOTiO; 8 Plasma sprnytil in argon. FBOTlOz .h' Do. A1 0; .8 Flame sprayed nt 3,00 l".

1 NhAl coated Tl) nickel or A1 0; containing NirAI coatcd TD nickel.

the use of a plurality of discrete coatings in which the number of coatings and compositions thereof can be varied over a wide range to achieve a transition from 0 percent to 100 percent thermal emissivity coatings. In one embodiment, this is accomplished with a plurality of coats of nickel aluminide and A1 0; with decreasing nickel aluminide content to produce a thermal emissivity material transition of from 0 percent to 100 percent.

Referring to FIG. 1, which illustrates one embodiment of the coating system of this invention, the surface of a metal substrate 11 is rendered chemically clean and the surface is roughened by grit blasting. A base coat 13 of w/o nickel aluminide (Ni Al) is then flame sprayed or plasma sprayed in an argon atmosphere onto the surface of the roughened substrate to form a coating 13 having a thickness of approximately 0.007 inches. In one example, the nickel aluminide is flame sprayed to a thickness of 0.004 inches on the metal substrate 1 l, e.g. TD nickel.

A second layer 15 of a composition having a major content, e.g. 65 w/o, of nickel aluminide and a minority content, e.g. 35 w/o, of emissivity material is then flame or plasma sprayed on top of the first coat 13. The thickness of this second coat l5 varies from about 0.002 to 0.0015 inches, and in the above described example is 0.002 inches thick. in this example, the coat 15 is a flame sprayed composition having a nickel aluminide content of 65 w/o and 35 w/o A1 0 Tests to 1,200 F. or above on these and other materials showed they were stable against thermal and handling shock. Other coats l3 and 15 were, for example, 0.007 inch 100 percent Ni,Al and plasma arc argon atmosphere sprayed 0.005 in. 65 percent Ni Al+35 percent Feo'liO which remained intact against thermal shock from 1,200 P. to room temperature. Likewise a plasma arc argon atmosphere sprayed CaOTiO, coating 13 remained intact under like thermal shock conditions.

A third layer 17 of a composition having a minority content, e.g. 35 w/o, of nickel aluminide and a majority content, e.g. 65 w/o, of the same thermal emissivity material is then flame or plasma sprayed to a thickness of between 0.002 and 0.015 inches on the surface of the second coat 15, the above mentioned example being flame sprayed to a thickness of 0.002 inches. In this example, the nickel aluminide content of coat 17 is 35 w/o nickel aluminide and 65 w/o A1 A final top coat 19 of 100 percent of the emissivity coating material is then flame or plasma sprayed to a thickness of up tofv 0,003 inches or less on the thirdcoat17. In the above cited example, flame sprayed 100 percent A1 0 is employed having a thickness of 0.002 inches.

The described coats are bonded to substrates ll of stainless steel, TD nickel, nickel alloys, such as lnconel 600, or Hastelloy, or Haynes 25, by a combination of an actual diffusion bond of the nickel, e.g. in the first coating and the substrate, plus a mechanical interlocking transition of the nonmetallic thermal emissivity material and nickel aluminide particles in the subsequent coatings from the base coating to the top coating. This is illustrated in FIG. 2. Moreover, the thermal emissivity material (e.g. A1 0 or Y O l (to 50) w/o Cr O can be merely physically blended to tailor the emissivity characteristics of the coating. Thus the weight percent (w/o) transition can be achieved in smaller or larger increments than the described example and combination of the various emissivity compounds can be used to achieve a wide emissivity spectrum.

In actual practice, the above-described four layer coating technique has successfully produced oxidation resistant high thermal emissivity coatings using nickel aluminide and ALO, as the emissivity material, while providing high thermal shock and handling shock resistance up to 2,000 F. and down to room temperature. For example, thermal emissivities E of up to 0.8 were measured using Martin Company test apparatus (Gier Dunkel) at room temperature with correction for operating temperature up to about 2,000 F. and a total hemispherical emissivity measuring device up to 2,000 F. experimentally. Moreover, any of the various respective layers can -be patched with its corresponding coating layer composition if metallic substrates for SNAP and other systems This method, moreover, provides high thermal and handling shock resistance, provides an easily patched emissivity coating, and provides a wide emissivity spectrum.

What is claimed is:

l. The method of forming a rugged coating on a metallic substrate for dumping heat into space from a radioisotope heat source, comprising the steps of preparing the surface of said substrate by cleaning and roughening said surface, coating the prepared surface of said substrate with a first coat of nickel aluminide, and coating said first coat with additional coats forming a compositional gradient from respectively decreasing amounts of nickel aluminide and correspondingly increasing amounts of a material selected from the group consisting of A1 0 CaOTiO,, Iridium, FeOTiO MO, and CaTiO 2. The invention of claim 1 including the steps of flame spraying a first coat of percent nickel aluminide 0.004 inches thick on said substrate, flame spraying a second coat on said first coat of 65 w/o nickel aluminide and 35 w/o Al,0, having a thickness of 0.002 inches, flame spraying a third coat on said second coat of 35 w/o nickel aluminide and 65 w/o A1 0 having a thickness of 0.002 inches, and flame spraying a top coat on said third coat of 100 w/o A1 0 having a thickness of 0.002 inches. 

2. The invention of claim 1 including the steps of flame spraying a first coat of 100 percent nickel aluminide 0.004 inches thick on said substrate, flame spraying a second coat on said first coat of 65 w/o nickel aluminide and 35 w/o A12O3 having a thickness of 0.002 inches, flame spraying a third coat on said second coat of 35 w/o nickel aluminide and 65 w/o A12O3 having a thickness of 0.002 inches, and flame spraying a top coat on said third coat of 100 w/o A12O3 having a thickness of 0.002 inches. 